Conventionally, most GaN-based semiconductor devices that include Light Emitting Diode (LED), Laser Diode (LD), Hetero-junction Bipolar Transistor (HBT), High Electron Mobility Transistor (HEMT), are fabricated using insulating sapphire substrate. As a result, device structures constructed with insulating substrate are typically constructed into lateral structures since a top side n-contact must be formed to make an electrical connection with the top side p-contact.
This construction causes numerous device performance problems such as current crowding and weak resistance to electrostatic discharge (ESD). The current crowding can become critical when high current injection is required for lighting applications using high power white LEDs or a blue/UV LD. Since the electrons are confined near the n-type electrode in such devices, the photon generation in the opto-electronic devices is limited with respect to increased current injection. In other words, the power efficiency suffers. This is a critical drawback of lateral devices currently available in the market.
The ESD issue is considered a serious problem, particularly when GaN-based LEDs are employed in a high voltage environment, for example, in automobile applications. Once electrostatic charge occurs on the device surface, the lateral device experiences charge build up which often leads to device failure within very short period since there is no current discharge path in the device due to the insulating substrate.
The other critical disadvantage of lateral devices having an insulating substrate like sapphire is the poor heat dissipation. Sapphire is known to be a poor heat conductor. Hence, the device lifetime is significantly shortened when the device is subjected to a high current injection mode. These are two are critical hurdles for the further development of GaN-based LEDs and LDs, and blue/UV LDs.
From the production yield point of view, the lateral structure device also has numerous disadvantages. Devices constructed with lateral structures need large device dimensions because both the p and n electrode are placed in the same plane as shown in FIG. 1. Hence the number of devices is limited due to the amount of wafer real estate the lateral devices require.
In addition to the issues raised above, sapphire substrate material is known to be the second hardest material, next to diamond. This causes difficulty in wafer grinding and polishing. Moreover, it is also difficult to separate the devices from the wafer. Therefore, even though one can expect high device yield rate up to front fabrication processes, the ultimate device fabrication yield is mainly dependent on post fabrication processes that include lapping, polishing, and die separation.
Recently, there have been new developments concerning a vertical structure GaN-based compound semiconductor, depicted in FIG. 2. A laser lift-off process has been introduced to remove the sapphire substrate from the GaN epi layer. Some techniques have substituted the insulating sapphire substrate with a conductive or semi-conductive second substrate to fabricate vertical structure devices using an excimer laser with a wavelength transparent to sapphire, typically in the UV range. It is noted that most other techniques utilize wafer-bonding techniques for permanent bonding to the second substrate after removing sapphire substrate by laser lift-off.
However, these techniques have not resulted in a practical wafer scale laser lift-off process for the mass production of VLEDs (Vertical LED). The two main reasons are the difficulty in large area laser lift-off due to de-lamination of bonding adhesive layer between support wafer and the epitaxial layer. The other problem is the difficulty in wafer bonding between epitaxial layer and a permanent second substrate since the epitaxial layer surface is not flat on entire wafer surface after laser lift-off. Because of these reasons, the final yield after laser lift-off greatly hampered, as a result, only small fragment portion of wafers have been fabricated for vertical structure devices according to the other techniques.
There have been other efforts to overcome the wafer bonding problems to fabricate VLEDs. Instead using wafer bonding methods, one other technique shown in FIG. 3 attaches a metal support. However, the laser lift-off yield is known to be very low due to de-lamination of the bonding layer to the support structure. If the bonding is not secure enough to withstand the high-energy laser shock wave, the GaN epi layers may buckle or crack after laser lift-off. Once cracks or buckles exist on the GaN epi layer, it is very difficult to perform a post laser lift-off process, such as cleaning, de-bonding, and device separation. Hence, final device process yield becomes very low even though the other process yield can maintain very high. These problems are mainly attributed to the temporary wafer bonding technique and non-optimized laser processing technique used.
Another problem with conventional vertical devices based on another technique, shown in FIG. 3, is poor device performance. Since sand blasting is often used on the sapphire substrate to create a uniform laser beam energy distribution, the GaN surface after laser lift-off is very rough, which results in poor reflectivity of the device. In addition, the metal reflective layer formed on the n-GaN layer is not as high as non-metallic reflector material, such as ITO.
What is needed is a method of fabricating vertical structure compound semiconductor devices that provides a reliable and repeatable laser lift-off process while obtaining high device performance in order to apply laser lift-off process to the fabrication of vertical structure devices.